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Yes! Take the cycle path-y route out of Glasgow center (nice, along river, avoids all roads - gentle cruising rather than strava-busting) then take route 7 from Loch Lomand into the Trossachs) - beautiful cycling, beautiful scenery. Reckon you could split off route 7 at Callander (which is lovely btw and def worth a visit - some cracking pubs for afternoon ale stops) and either loop back to Glasgow if you wanted a 100 mile-ish circular, or aim for train station of your choice to A-B it. Definitely recommend continuing along route 7 as far as you can, all of it is amazing, and the A85 through Crianlarich is lovely too...
Edit: Route 7 link: http://www.sustrans.org.uk/ncn/map/route/route-7
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What sort of distance do you want? Fancy hills or flatter rides? Whereabouts are you staying?
Personally, I don't think you can go far wrong in heading north into the Campsie Fells. Stunning scenery and quiet roads. Loch Lomond is beautiful and you can see the Highland Fault run through it, and, as @owl says, the Trossachs are well worth a visit too.
Heading west into Renfrewshire and out towards the Ayrshire coast is good too.
For a big day out you could ride the route of the long standing Scottish classic road race, Glasgow-Dunoon. Head up Loch Lomond side, turn left at Tarbet, through Arrochar, over the Rest And Be Thankful, then turn left onto the A815 just before Cairndow and ride down to Dunoon. Get the ferry across to Gourock and ride back to Glasgow.
Anyone know of any particularly good routes starting around Glasgow (I know)?
Spending a week there with my girlfriend next week and will be riding my bike while she does her job.